Re: [PATCH v2 0/8] repack: introduce `--write-midx`

From: Taylor Blau <me@ttaylorr.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Taylor Blau <me@ttaylorr.com>,
	git@vger.kernel.org, peff@peff.net, avarab@gmail.com
Subject: Re: [PATCH v2 0/8] repack: introduce `--write-midx`
Date: Wed, 15 Sep 2021 17:19:12 -0400	[thread overview]
Message-ID: <YUJjUIEmIFTH0+jq@nand.local> (raw)
In-Reply-To: <xmqqilz1wsbz.fsf@gitster.g>

On Wed, Sep 15, 2021 at 12:29:20PM -0700, Junio C Hamano wrote:
> Junio C Hamano <gitster@pobox.com> writes:
>
> > But if it is the case, I'd step back a bit and further question if
> > "else if" is a good construct to use here.  We'd return if .m passes
> > midx_contains_pack() check, and another check based on .to_include
> > gives us an orthogonal chance to return early, so two "if" statement
> > that are independent sitting next to each other may have avoided
> > such a bug from the beginning, perhaps?
>
> OK, I went back and checked your response to a review in an earlier
> round.  If .m and .to_include cannot be turned on at the same time,
> then I think "else if" would express the intention more clearly.

Yeah, exactly. I didn't think that this would be as interesting to
reviewers as it ended up being, hence I didn't put much thought into it.

> But if we go that route, the whole "if ... else if" may deserve a
> comment that explains why .m and .to_include are fundamentally and
> inherently mutually exclusive.  In other words, is it possible if
> future enhancement may want to pass both .m and .to_include to allow
> the code path to check both conditions and return early?

The gist is that they aren't inherently exclusive, but that using an
existing MIDX (which is the result of having `.m` point at a MIDX
struct) right now blindly reuses all of the existing packs in that MIDX,
which is what to_include is trying to avoid.

We could reuse an existing MIDX with to_include by filtering down its
packs before carrying them forward, but don't currently. So that would
cause this change to produce unexpected results if we ever changed that
behavior.

I think all of this is non-obvious enough that it warrants being written
down in a comment. Here's a replacement for that first patch that should
apply without conflict. But if you'd rather have a reroll or anything
else, I'm happy to do that, too.

--- >8 ---

Subject: [PATCH] midx: expose `write_midx_file_only()` publicly

Expose a variant of the write_midx_file() function which ignores packs
that aren't included in an explicit "allow" list.

This will be used in an upcoming patch to power a new `--stdin-packs`
mode of `git multi-pack-index write` for callers that only want to
include certain packs in a MIDX (and ignore any packs which may have
happened to enter the repository independently, e.g., from pushes).

Those patches will provide test coverage for this new function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++---------
 midx.h |  5 +++++
 2 files changed, 55 insertions(+), 9 deletions(-)

diff --git a/midx.c b/midx.c
index 864034a6ad..61226eaa9d 100644
--- a/midx.c
+++ b/midx.c
@@ -475,6 +475,8 @@ struct write_midx_context {
 	uint32_t num_large_offsets;

 	int preferred_pack_idx;
+
+	struct string_list *to_include;
 };

 static void add_pack_to_midx(const char *full_path, size_t full_path_len,
@@ -484,8 +486,26 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,

 	if (ends_with(file_name, ".idx")) {
 		display_progress(ctx->progress, ++ctx->pack_paths_checked);
+		/*
+		 * Note that at most one of ctx->m and ctx->to_include are set,
+		 * so we are testing midx_contains_pack() and
+		 * string_list_has_string() independently (guarded by the
+		 * appropriate NULL checks).
+		 *
+		 * We could support passing to_include while reusing an existing
+		 * MIDX, but don't currently since the reuse process drags
+		 * forward all packs from an existing MIDX (without checking
+		 * whether or not they appear in the to_include list).
+		 *
+		 * If we added support for that, these next two conditional
+		 * should be performed independently (likely checking
+		 * to_include before the existing MIDX).
+		 */
 		if (ctx->m && midx_contains_pack(ctx->m, file_name))
 			return;
+		else if (ctx->to_include &&
+			 !string_list_has_string(ctx->to_include, file_name))
+			return;

 		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);

@@ -1058,6 +1078,7 @@ static int write_midx_bitmap(char *midx_name, unsigned char *midx_hash,
 }

 static int write_midx_internal(const char *object_dir,
+			       struct string_list *packs_to_include,
 			       struct string_list *packs_to_drop,
 			       const char *preferred_pack_name,
 			       unsigned flags)
@@ -1082,10 +1103,17 @@ static int write_midx_internal(const char *object_dir,
 		die_errno(_("unable to create leading directories of %s"),
 			  midx_name);

-	for (cur = get_multi_pack_index(the_repository); cur; cur = cur->next) {
-		if (!strcmp(object_dir, cur->object_dir)) {
-			ctx.m = cur;
-			break;
+	if (!packs_to_include) {
+		/*
+		 * Only reference an existing MIDX when not filtering which
+		 * packs to include, since all packs and objects are copied
+		 * blindly from an existing MIDX if one is present.
+		 */
+		for (cur = get_multi_pack_index(the_repository); cur; cur = cur->next) {
+			if (!strcmp(object_dir, cur->object_dir)) {
+				ctx.m = cur;
+				break;
+			}
 		}
 	}

@@ -1136,10 +1164,13 @@ static int write_midx_internal(const char *object_dir,
 	else
 		ctx.progress = NULL;

+	ctx.to_include = packs_to_include;
+
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
 	stop_progress(&ctx.progress);

-	if (ctx.m && ctx.nr == ctx.m->num_packs && !packs_to_drop) {
+	if ((ctx.m && ctx.nr == ctx.m->num_packs) &&
+	    !(packs_to_include || packs_to_drop)) {
 		struct bitmap_index *bitmap_git;
 		int bitmap_exists;
 		int want_bitmap = flags & MIDX_WRITE_BITMAP;
@@ -1237,7 +1268,7 @@ static int write_midx_internal(const char *object_dir,

 	QSORT(ctx.info, ctx.nr, pack_info_compare);

-	if (packs_to_drop && packs_to_drop->nr) {
+	if (ctx.m && packs_to_drop && packs_to_drop->nr) {
 		int drop_index = 0;
 		int missing_drops = 0;

@@ -1380,7 +1411,17 @@ int write_midx_file(const char *object_dir,
 		    const char *preferred_pack_name,
 		    unsigned flags)
 {
-	return write_midx_internal(object_dir, NULL, preferred_pack_name, flags);
+	return write_midx_internal(object_dir, NULL, NULL, preferred_pack_name,
+				   flags);
+}
+
+int write_midx_file_only(const char *object_dir,
+			 struct string_list *packs_to_include,
+			 const char *preferred_pack_name,
+			 unsigned flags)
+{
+	return write_midx_internal(object_dir, packs_to_include, NULL,
+				   preferred_pack_name, flags);
 }

 struct clear_midx_data {
@@ -1660,7 +1701,7 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
 	free(count);

 	if (packs_to_drop.nr) {
-		result = write_midx_internal(object_dir, &packs_to_drop, NULL, flags);
+		result = write_midx_internal(object_dir, NULL, &packs_to_drop, NULL, flags);
 		m = NULL;
 	}

@@ -1851,7 +1892,7 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 		goto cleanup;
 	}

-	result = write_midx_internal(object_dir, NULL, NULL, flags);
+	result = write_midx_internal(object_dir, NULL, NULL, NULL, flags);
 	m = NULL;

 cleanup:
diff --git a/midx.h b/midx.h
index aa3da557bb..80f502d39b 100644
--- a/midx.h
+++ b/midx.h
@@ -2,6 +2,7 @@
 #define MIDX_H

 #include "repository.h"
+#include "string-list.h"

 struct object_id;
 struct pack_entry;
@@ -62,6 +63,10 @@ int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
 int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local);

 int write_midx_file(const char *object_dir, const char *preferred_pack_name, unsigned flags);
+int write_midx_file_only(const char *object_dir,
+			 struct string_list *packs_to_include,
+			 const char *preferred_pack_name,
+			 unsigned flags);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(struct repository *r, const char *object_dir, unsigned flags);
 int expire_midx_packs(struct repository *r, const char *object_dir, unsigned flags);
--
2.33.0.96.g73915697e6

